The recent FDA approvals of abiraterone and enzalutamide, two new agents targeting the androgen receptor (AR) signaling axis, have expanded the treatment options and improved outcomes for patients with castration-resistant prostate cancer (CRPC). These drugs have dramatically changed the therapeutic landscape of CRPC even though a significant proportion of patients do not derive any clinical benefit from them. Thus, there has emerged a critically unmet need to address the mechanisms of resistance to these agents. The overall objective of this application is to dissect molecular drivers of therapeutic resistance to abiraterone and enzalutamide. The main hypothesis of this application is that aberrant AR signaling, mediated by the constitutively active AR splice variants, underlies primary and acquired resistance to abiraterone and enzalutamide. The proposed studies will be conducted in a translational setting, involving prospective collection of clinically annotated specimens from patients with metastatic CRPC undergoing standard-of-care treatment with abiraterone or enzalutamide. Three Specific Aims are proposed: (Aim 1) to determine whether blood-based detection of a key AR splice variant, AR-V7, indicates primary or acquired resistance to abiraterone or enzalutamide; (Aim 2) to define the transcriptional landscape of the aberrant AR in a comprehensive fashion, with the goal of determining the relative importance and clinical significance of AR-Vs in drug response and resistance, in the context of other putative competing resistance mechanisms; and (Aim 3) to determine the role of AR dimers in mediating aberrant AR signaling. If successful, this project will determine the potential clinical utility of a non-invasive assay for AR-V7, will define the relative importance ad clinical significance of a number of competing resistance mechanisms, and will establish AR-Vs as drivers of resistance to AR-targeting agents. Overall, the proposed studies have the potential to transform the clinical management of metastatic CRPC and will help to drive the development of the next wave of approaches targeting the aberrant AR signaling axis.